The present invention relates to subterranean stimulation operations and, more particularly, to methods of stimulating a subterranean formation comprising multiple production intervals.
To produce hydrocarbons (e.g., oil, gas, etc) from a subterranean formation, well bores may be drilled that penetrate the hydrocarbon-containing portions of the subterranean formation. The portion of the subterranean formation from which hydrocarbons may be produced is commonly referred to as a “production interval.” In some instances, a subterranean formation penetrated by the well bore may have multiple production intervals at various depths in the well bore.
Generally, after a well bore has been drilled to a desired depth completion operations may be performed. Completion operations may involve the insertion of casing into a well bore, and thereafter the casing, if desired, may be cemented into place. So that hydrocarbons may be produced from the subterranean formation, one or more perforations may be created that penetrate through the casing, through the cement, and into the production interval. At some point in the completion operation, a stimulation operation may be performed to enhance hydrocarbon production from the well bore. Stimulation operations may involve hydraulic fracturing, acidizing, fracture acidizing, or other suitable stimulation operations. Once the stimulation operation has been completed and after any intermediate steps, the well bore may be placed into production. Generally, the produced hydrocarbons flow from the production intervals, through the perforations that connect the production intervals with the well bore, into the well bore, and to the surface.
Stimulation operations such as these may be problematic in subterranean formations comprising multiple production intervals. In particular, problems may result in stimulation operations where the well bore penetrates multiple perforated and depleted intervals due to the variation of fracture gradients between these intervals. The most depleted intervals typically have the lowest fracture gradients among the multiple production intervals. When a stimulation operation is simultaneously conducted on all of the production intervals, the treatment fluid may preferentially enter the most depleted intervals. Therefore, the stimulation operation may not achieve desirable results in those production intervals having relatively higher fracture gradients. Packers and/or bridge plugs may be used to isolate the particular production interval before the stimulation operations, but this may be problematic due to the existence of open perforations in the well bore and the potential sticking of these mechanical isolation devices.
Another method conventionally used to combat problems encountered during the stimulation of a subterranean formation having multiple production intervals has been to perform a remedial cementing operation prior to the stimulation operation to plug the open perforations in the well bore, thereby hopefully preventing the undesired entry of the stimulation fluid into the most depleted intervals of the well bore. Once the pre-existing perforations are plugged with cement, a particular production interval may be perforated and then stimulated. While these remedial cementing operations may plug some of the pre-existing perforations and thus reduce the entry of the stimulation fluid into undesired portions of the formation, remedial cementing operations may not be completely effective in plugging all the pre-existing perforations in the well, requiring multiple remedial cementing operations to ensure complete plugging of all the pre-existing perforations. Further, remedial cementing operations may damage near well bore areas of the subterranean formation and/or require further remedial operations to remove undesired cement from the well bore before the well may be placed back into production.